Photoelectric smoke detectors have been recognized as being useful in providing signals indicative of concentrations of smoke or particles of combustion in the ambient atmosphere. Such detectors can be used alone or in groups to provide an indication of a developing fire condition.
Known photoelectric smoke detectors often provide circuitry for testing the respective detector. Various types of test circuitry are known.
The graph of FIG. 1 contains 2 curves, i.e. curve A and curve B. The units for smoke concentration and radiation sensor signal appear in arbitrary units; the ranges of values are chosen for illustration.
Curve A depicts a typical photoelectric smoke detector's radiation sensor output as a function of smoke concentration. In the absence of smoke (smoke concentration=0), the radiation sensor generates a nonzero output (shown 0.2) resulting from background reflections of radiation inside the smoke detection chamber. The reflected radiation originates from the internal radiation source, reflects from the inside walls of the chamber, and finally irradiates the radiation sensor to produce a nonzero output.
A known "self test" technique employs a higher radiation sensor amplifier gain during a "self test" mode, so that the amplifier output simulates the presence of smoke within the detection chamber. For example, a "test" gain whose magnitude is greater than "normal mode" gain by a factor of 6 would exceed an alarm threshold corresponding to a smoke concentration of 1 in the absence of smoke. This follows since six times the 0.2 radiation sensor signal yields a signal of 1.2. In "normal mode", the detector requires a smoke concentration of 1.0 to cause a radiation sensor signal of 1.2.
Curve B of FIG. 1 depicts a photoelectric smoke detector's radiation sensor output, when the optics employ a tightly focused laser diode, a radiant energy source, specifically arranged to minimize unwanted background reflections. In the absence of smoke (smoke concentration=0), the radiation sensor generates a zero output, or an output very small in magnitude. Such a small radiation sensor output renders the above described "self test mode" smoke simulation technique problematic or even nonfunctional.
One known solution to the "self test" problem inherent in low background noise photoelectric detectors utilizes a separate "test" radiation source to directly or indirectly irradiate the sensor. Such schemes fail to assess the proper operation of the "normal" radiation source, i.e. the laser diode.
There continues to be a need for circuitry and methods of testing low background noise photoelectric smoke detectors which can also take into account the level of functioning of the radiant energy source for the detector. Preferably, such circuitry could be incorporated into low background noise photoelectric detectors without undue expense and without detracting in any way from the performance of such detectors.